Critical temperature of superconducting bilayers: Theory and experiment

A generalized model for the critical temperature TC of superconducting bilayers is presented, which is valid with no restrictions to film thicknesses, TC of the layers, and interface resistivity. The model is verified experimentally on a series of Nb–Al and Ta–Al bilayers with Nb, Ta layer thicknesses of 100 nm and Al layer thicknesses ranging from 5 to 200 nm. Excellent agreement between theory and experiment was found for the energy gap and the TC of bilayers. The results are important for designing practical superconducting devices.